Frozen
reservoir fuels atom lasers

By
Eric Smalley,
Technology Research NewsLaser beams made of matter rather than
light are a step closer to reality now that researchers at the Massachusetts
Institute of Technology have figured out how to continuously replenish
a reservoir of ultra-cold atoms.

The technique merges tiny clouds of atoms that are so cold they behave
as a single entity. These Bose-Einstein Condensates are the main components
of atom lasers. Prototype atom lasers have, to date, worked only as pulsed
rather than continuous beams.

Pulsed atom lasers are analogous to a dripping faucet, said Ananth Chikkatur,
a researcher at MIT. "We have now implemented a bucket -- our reservoir
trap -- where we collect these drips to have a continuous source of water.
If we poke a hole in this bucket, we will have a steady stream of water,"
he said.

Atom lasers could be used to deposit matter on a surface atom by atom
to, for instance, produce very fine wires on a computer chip. They could
also make extremely sensitive movement sensors because atom waves, like
light waves, can interfere with each other, and the interference patterns
are affected by subtle changes in acceleration and gravity.

The project's leader, MIT physics professor Wolfgang Ketterle, was one
of the recipients of the 2001 Nobel Prize in physics, which was awarded
for the discovery of Bose-Einstein Condensates.

One of the strange properties of quantum particles like atoms and photons
is that they also act like waves. In a Bose-Einstein Condensate, the crests
and troughs of the atoms' waves are in sync, much like the photons in
a laser beam.

Getting the atoms to snap into this quantum lockstep requires cooling
them to a fraction of a degree above absolute zero. Forming Bose-Einstein
Condensates is a two-step cooling process involving lasers and evaporation.

The hotter matter is, the faster its atoms move, though not all of the
atoms move at the same speed. The researchers used a laser tuned to send
a stream of photons into the paths of the fastest moving, and therefore
hottest, atoms in a gas. The impact transferred energy from the atoms
to the photons, slowing and thus cooling the atoms.

In the second step of the cooling process, the researchers held the atoms
in a trap formed by a magnetic field, and then gradually diminished the
strength of the trap to allow the hottest atoms to escape.

The researchers formed a Bose-Einstein Condensate consisting of about
2 million sodium atoms. They then formed another condensate and added
it to the first, which lost half of its atoms in the time it took to produce
the second condensate. The merged condensate totaled 2,300,000 atoms.

The researchers move the condensates using laser tweezers. When a laser
beam shines through a small, transparent object the light bends, which
transfers momentum to the object, much like wind moving the vanes of a
windmill. This force can also be used to hold an object within a laser
beam.

Bose-Einstein Condensates are very fragile and the challenge is being
able to merge them without heating them up, said Chikkatur. The condensates
held in the laser beams were elongated, and the researchers found that
gently lowering one condensate lengthwise on top of the other did the
trick, he said.

Researchers have already developed several techniques for draining Bose-Einstein
Condensates to form atom lasers. "By combining [output] techniques using
optical laser beams with our continuous source, we [will be able to] generate
a continuous beam of coherent atoms," said Chikkatur.

The researchers' next step is to increase the number of atoms collected
in the reservoir, said Chikkatur.

The researchers' work is a "tour de force and a major step forward in
the technology of Bose-Einstein condensation," said Aephraim Steinberg,
an associate professor of physics at the University of Toronto. "It is
extremely promising for the development of real continuous-wave atom lasers,"
he said.

In the short-term, continuous-wave atom lasers will allow scientists to
study how quantum particles change, or decohere, when they come into contact
with their environment, said Steinberg.

Isolated atoms and subatomic particles are in the weird quantum mechanical
condition of superposition, an unknowable mix of all possible orientations.
When energy from the environment, like a stray magnetic field, hits a
particle and knocks it out of superposition, it resumes one, definite
orientation. "Decoherence [is] one of the processes defining the boundary
between quantum and classical, and one of the important obstacles to overcome
if we are to develop quantum computers," said Steinberg.

It is impossible to predict whether atom lasers will have direct technological
applications, said Steinberg. "We're more or less in the situation of
laser researchers in 1960 who could never have envisioned the applications
lasers have today," he said.

It will take five to ten years for continuous-wave atom lasers to be used
to deposit atoms on a surface in practical applications, said Chikkatur.
"For atom lithography one needs to have a very high [flow rate] of atoms,
which is not possible currently," he said.

Chikkatur's research colleagues were Yong-Il Shin, Aaron E. Leanhardt,
David Kielpinski, Edem Tsikata, Todd L. Gustavson, David E. Pritchard
and Wolfgang Ketterle. They published the research in the May 16, 2002
issue of the online issue of the journal Science. The research was funded
by the National Science Foundation (NSF), the Office of Naval Research
(ONR), the Army Research Office (ARO), the Packard Foundation and NASA.